Citable URI

As published

DOI

Keyword

Abstract

One process for the formation of continental crust is the accretion of arc
terranes at continental margins. A longstanding problem with this model is that although
the composition of the continental crust is andesitic, the majority of arc lavas are basaltic.
Moreover, those arc lavas that are andesitic tend to be evolved (lower Mg #) compared to
the continental crust. Continental crust can be produced through mixing of basaltic and
silicic arc lava compositions, assuming that mafic cumulates formed during generation of
the silicic component are removed. If these cumulates are denser than the underlying
mantle, removal can occur via foundering of lower arc crust. Indeed, field observations
of the Talkeetna arc section in south-central Alaska, combined with modeling of
fractionation in primitive arc magmas, suggest that large amounts of primitive
gabbronorite and pyroxenite are missing from the lower crust. Using rock compositions
from the Talkeetna section and the free energy minimization program Perple_X, we
calculated equilibrium mineral assemblages for a range of gabbroic and ultramafic
compositions at P, T, oxygen fugacity (fO2), and H2O contents appropriate for arc lower
crust. The quartz-olivine-garnet-free mineral assemblage found in the Talkeetna
gabbronorites (and in the similar Kohistan section in Pakistan) defines a narrow range of
fO2 centered on NNO+2 (±1 log unit). Predicted mineral assemblages calculated under
these conditions were used to estimate the density and seismic structure of the arc lower
crust. We find that the missing gabbroic and ultramafic rocks from the Talkeetna section
were likely denser than the underlying mantle, while the gabbronorites that remain are
either neutrally or slightly positively buoyant. Generalizing, we show that lower crustal
Vp > 7.4 km/s in modern arcs is indicative of lower crust that is convectively unstable
relative to the underlying mantle. However, most lower crust in modern arcs is observed
to have Vp < 7.4 km/s, implying that gravitationally unstable material must founder
rapidly on geologic time-scales, or high Vp plutonic rocks crystallize beneath the Moho.

This workshop was convened to follow up on the Second Conference on Scientific Ocean Drilling (COSOD II) to devise a
specific plan for deep crustal and mantle drilling over the next decade. Since COSOD II, however, there ...

Determining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust. We present new constraints on the ...

The oceanic crust extends over two thirds of the Earth’s solid surface and is
generated along mid-ocean ridges from melts derived from the upwelling mantle.
The upper and mid crust are constructed by dyking and seafloor ...

All Items in WHOAS are protected by original copyright, with all rights reserved, unless otherwise indicated. WHOAS also supports the use of the Creative Commons licenses for original content.
A service of the MBLWHOI Library |
About WHOAS